(1) Field of the Invention
The present invention relates to the general technical field of de-icing aircraft structures, and in particular fixed or rotary wings.
(2) Description of Related Art
The invention relates more particularly to de-icing by ultrasound, i.e. transmitting ultrasonic mechanical vibration over a structure that is to be de-iced and/or, where appropriate, to be cleaned. In addition to being covered in ice, the above-mentioned structures might equally well be covered in a deposit of dust that needs to be removed.
The present invention is described mainly with reference to aircraft of the rotorcraft type, and in particular to helicopters, however it is applicable to aircraft of all types, whether fixed wing or rotary wing.
Various de-icer technologies already exist. Thus, electrothermal de-icers are known that are used on helicopters. Such de-icers comprise electrical resistances embedded in the leading edges of the blades. The electrical resistances use the Joule effect to convert electrical energy that is supplied to them into heat. The heat serves to raise the temperature of the leading edge above temperatures that are favorable for ice formation.
Nevertheless, those de-icers present certain drawbacks, in particular for application to rotorcraft of the helicopter kind. Some de-icers consume large amounts of electrical power, for example power lying in the range 10 kilowatts (kW) to 40 kW. Such de-icers are also heavy and bulky because of the large amount of electrical power that is needed for their operation. Under such conditions, such de-icers cannot be installed on small machines. Furthermore, de-icers require the use of numerous parallel slip rings and brushes. Such de-icers therefore constitute systems that are complex and very expensive.
Such de-icers are also made heavier because of their control portions, i.e. the electronics used for controlling them, which electronics occupy a non-rotary frame of reference.
Furthermore, in the event of the electrical temperature control drifting, such de-icers can lead to the temperature drifting and consequently to the leading edge delaminating. Naturally it is possible to counter that problem by using a thermal protection device embedded in the blade. However such an additional protection device significantly increases the cost of the de-icer and also its on-board weight, which is not desirable.
Ultrasound de-icers are also known that serve to de-ice a structure by applying shear to the ice by means of said ultrasound. An example of the general principle of how such de-icers operate is described for example in document US 2010/0031972. That document describes in particular the use of piezoelectric actuators and also their modes of excitation in order to maximize effectiveness in transmitting ultrasonic waves without affecting the structure that is to be de-iced.
Such de-icers, e.g. based on ceramic elements, present the drawback of not being suitable for being incorporated in structures such as the leading edges of helicopter blades. Leading edges are generally mechanically fastened integrally and in very rigid manner onto a base structure, and the adhesives used for holding said leading edges in place block the vibrating elements and thus prevent them from transmitting their energy. Arranging de-icers in the vicinity of surfaces for de-icing is also made difficult by the fact that there is little space available under the leading edge.
Another drawback of ultrasound de-icers is associated with incorporating them in blades. Loose wiring used for making connections to such actuators gives rise to uncontrolled volumes of air and also to zones in which the adhesive or the resin does not polymerize. Exothermic portions may also appear during fabrication of blades.
All of those drawbacks increase the risk of a leading edge being lost in flight, thereby constituting a situation that is catastrophic for a helicopter.
Furthermore, it should be observed that the ceramic elements used in known ultrasound de-icers need to be prestressed.
The ceramic elements are prestressed in order to protect them from certain forces to which they are particularly sensitive (twisting type forces and more generally any forces off their main axes). Furthermore, prestress does not enable them to exceed their mechanical limits in operation, and the ceramic elements may be damaged if those limits are exceeded.